A typical laser radar device as disclosed in the following Patent Document 1 radiates laser light that is pulsed light in the atmosphere, and then to receive laser light (scattered light) that is reflected by an aerosol existing in the atmosphere to be returned.
The laser radar device performs heterodyne detection on the scattered light and transmitted pulsed light to find a Doppler shift that occurs with the movement of the aerosol, and the wind speed in a laser irradiation direction (moving speed of the aerosol) is measured based on the Doppler shift.
FIG. 9 is a configuration diagram depicting a typical laser radar device. The laser radar device includes the following components:
(1) a light source 101 that oscillates continuous wave light (laser light) with a single frequency, referred to as local light;
(2) a light distributor 102 that distributes the continuous wave light oscillated from the light source 101 into two beams, and outputs one continuous wave light to a pulse modulator 103 and outputs the other continuous wave light to an optical coupler 106;
(3) the pulse modulator 103 that gives a predetermined frequency shift to the continuous wave light outputted from the light distributor 102, and also performs pulse modulation on the continuous wave light whose frequency has been shifted and outputs the pulsed light to an optical circulator 104;
(4) the optical circulator 104 that outputs the pulsed light outputted from the pulse modulator 103 to an optical antenna 105, and outputs scattered light received at the optical antenna 105 to the optical coupler 106;
(5) the optical antenna 105 that radiates the pulsed light outputted from the optical circulator 104 in the atmosphere, and then receives the scattered light (pulsed light) that is reflected by the aerosol existing in the atmosphere to be returned; and
(6) the optical coupler 106 that multiplexes the continuous wave light outputted from the light distributor 102 with the scattered light outputted from the optical circulator 104, and outputs an optical signal of the multiplexed light to a light receiver 107;
(7) the light receiver 107 that performs heterodyne detection on the optical signal outputted from the optical coupler 106 to convert the optical signal into an electrical signal, and outputs the electrical signal to an analog/digital (A/D) converter 108;
(8) the A/D converter 108 that converts the electrical signal outputted from the light receiver 107 from an analog signal to a digital signal;
(9) a fast Fourier transform (FFT) device 109 that performs frequency analysis on the digital signal outputted from the A/D converter 108 by a unit of an FFT gate (range gate) with a fixed width to calculate the spectrum of the digital signal;
(10) a frequency shift analysis device 110 that calculates an amount of frequency shift that occurs with the movement of the aerosol based on the spectrum calculated by the FFT device 109; and
(11) a wind speed conversion device 111 that converts a wind speed (moving speed of the aerosol) in a laser irradiation direction from the frequency shift amount calculated by the frequency shift analysis device 110.
As described above, the laser radar device is configured such that the FFT device 109 performs frequency analysis on the digital signal outputted from the A/D converter 108 by the FFT gate unit with the fixed width to calculate the spectrum of the digital signal, while in order to enhance an SNR (Signal to Noise ratio), it is necessary that a longer width of the FFT gate be set.
However, since a coherence length changes due to a factor such as fluctuation in environmental conditions, the coherence length is shorter than the width of the FFT gate in some cases.
Now, FIGS. 10(a) and 10(b) are explanatory diagrams for illustrating heterodyne efficiencies in case of the coherence length longer than the width of the FFT gate (range gate) and in case of the coherence length shorter than the width thereof.
FIG. 10(a) depicts a case of the coherence length longer than the width of the FFT gate, where the amplitude of signal components is sufficiently larger as compared to the amplitude of noise components, and the heterodyne efficiency is thus higher.
FIG. 10(b) depicts a case of the coherence length shorter than the width of the FFT date, where the amplitude of signal components is not sufficiently larger as compared to the amplitude of noise components, and the heterodyne efficiency is thus lower.